ABOVE: The pentaprism assembly of an SLR camera, which allows you to view what the camera sees through the lens. The focusing screen is the flat screen with red markings.
The eyepiece is a small magnifying lens and it is this which enables you to focus your eye on the screen. This is crucial, as otherwise you wouldn’t be able to focus on anything that close to your face.
Additionally, because the image is formed on the focusing screen, this means that your eye doesn’t need to change focus because you are constantly looking at a fixed point, regardless of which area of the frame you are looking at.
Digital and film SLR cameras come factory-fitted with a focusing screen that has few, if any, markings. It is designed to give you a clear view of the subject and your camera’s autofocus points, with a reasonable balance between viewfinder brightness and manual focusing capability – Canon call it a standard precision matte focusing screen.
There are times, though, when some markings in the viewfinder could help you out. The most common situation is when photographing landscapes or architecture – a grid in the viewfinder would help you keep horizons and buildings straight. Some EOS cameras offer the option for interchangeable focusing screens, meaning that you can opt for a screen which offers markings.